1. Field of the Invention
The present invention relates to an inkjet printing apparatus for forming an image on a sheet.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2003-326680 discusses a technique in which a sheet is heated by a heater after ink is applied, to promote dryness of the sheet. The technique controls the amount of heating by increasing or decreasing the temperature of a heater according to the duty of image recording (hereinafter referred to as duty), that is, the amount of ink applied to the sheet per unit area.
An application in which a large-format advertizing poster displayed outdoor is produced by using an ink-jet printing apparatus has attracted attention. In this application, printing is performed by applying ink to a sheet high in weather resistance, such as sheet of vinyl chloride. A recording medium in a sheet form without an ink receiving layer has the property that repels water without absorbing it. For this reason, it is difficult to perform printing using a general water-based ink by a system in which ink is fixed by air drying.
For printing on a sheet without an ink receiving layer, a method is conceivable in which the ink immediately after being impacted on a print surface is heated by a heater to evaporate moisture, increasing the viscosity of the ink. An investigation is made as to whether heating control can be applied in printing on the sheet without an ink receiving layer according to the duty discussed in Japanese Patent Application Laid-Open No. 2003-326680 and the following problems to be solved are found.
(1) A first problem: Precise management of the amount of heating.
If the print surface is insufficiently heated by a heater, such a phenomenon called beading occurs that ink droplets adjacently impacted aggregate without moisture being sufficiently removed immediately after the ink is impacted to collapse an image. If the print surface is excessively heated by a heater, on the other hand, the sheet itself is expanded or contracted depending on a property of the sheet to cause damage such as creases to the sheet. Thus, for printing on a sheet without an ink receiving layer, strict temperature control on the print surface is required.
(2) A second problem: Decrease in temperature of platen and sheet due to heat of vaporization.
As illustrated in FIG. 4A, thermal energy (heat transfer from the sheet surface and heat radiation from the heater) is provided for the ink impacted on the print surface of the sheet 3 so that moisture is evaporated in a short time period. Since moisture hardly penetrates through the sheet without an ink receiving layer, the ink is deprived of its heat of vaporization when moisture of the ink evaporates, so that the temperature of the sheet 3 is lowered. Printing on a plurality of sheets is continued to lower also the temperature of the surface of the platen 2 supporting the sheet 3. The platen 2 deprives the sheet 3 to be newly supplied on the platen 2 of its temperature, so that the temperature of the sheet 3 is further lowered. As a result, a period of time required for impacted ink to evaporate is extended to dry ink droplets with the droplets further extended than an intended dot size, as illustrated in FIG. 4B. If adjacent ink droplets are mixed with each other, as illustrated in FIG. 4C, beading occurs to degrade image quality.
(3) A third problem: Follow-up delay of heater temperature.
The working temperature of the heater is 300° C. to 500° C., for example. There is a significant time lag from the heater starting drive to the heater reaching a target temperature. The use of such a heater inferior in control response causes the deficiency or excess of the heat quantity provided on the print surface not to provide uniform image quality. To avoid this, the print speed is compelled to be lowered according to the response of the heater. A heater which is small in thermal capacity and size is high in response. However, arranging a plurality of small heaters brings disadvantages in the cost and the assemblage of the apparatus. In particular, a heating area is very large in a large-format printer and a large number of small heaters are used to cover the area, thus making the disadvantages conspicuous.